The Molmer-Sorensen Gate
The Molmer-Sorensen gate entangles trapped ions through their shared motion, insensitive to the exact motional state, giving the field's highest fidelities.
Entangling through motion
Ions in a trap are individually addressed for internal-state control, but to entangle two of them they must share something: their collective vibration in the trap. The Molmer-Sorensen (MS) gate uses laser beams tuned symmetrically around a motional sideband to create a state-dependent force that entangles the ions' internal states while returning the motion to where it started.
Why it is robust
A key virtue is that, to leading order, the MS gate does not depend on the initial number of motional quanta. This means it does not require the ions to be cooled all the way to the motional ground state, only near it, and it is forgiving of small heating. That robustness is a major reason ion gates reach fidelities above 99.9 percent, among the best of any modality.
The mechanism
- Bichromatic light drives the pair via virtual excitation of a motional mode
- The motion traces a closed loop in phase space during the gate
- The enclosed area imparts a geometric phase that entangles the internal states
- The motion disentangles from the qubits at the gate's end
Scaling within a chain
Because all ions in a chain share the same motional modes, an MS gate can in principle entangle any pair, giving all-to-all connectivity. As chains grow, though, the mode spectrum crowds and gates slow, which is why large machines shuttle ions between zones or network separate traps.
The MS gate is the workhorse entangler for trapped-ion computing, valued for its high fidelity and its tolerance of imperfect motional cooling.